Chaos Theory has several improvements over its predecessors and as such is a fantastic game.
I loved Splinter Cell and Pandora Tomorrow, but if there was one thing I had to gripe about the two was that triggering one too many alarms would result in the mission failing. Happily Chaos Theory got rid of this one complaint - triggering alarms will result in enemies being more cautious and ready for combat, but you won't get kicked out like in the previous two games. In addition to this, alarms aren't triggered if you eliminate all the NPCs.
A knife has been added to Sam's arsenal for breaking locks and killing enemies. Sam can attack enemies melee-wise from any direction in this game (in the previous games, he could only knock them out from behind) thanks to the fact that this game uses ragdoll physics. You can choose between knocking enemies out or killing them when you grab them from behind.
You can choose one of three kits - a stealth kit, an assault kit and a 'Redding's Recommended' kit which chooses the equipment best suited for the mission. The pistol has an OCP now which lets you disable electronic surveillance equipment like cameras. The rifle can have attachments like shotgun, launcher or sniper rifle. The launcher has equipment like sticky shockers, airfoil rounds and sticky cameras. There are also grenades like frag grenades and gas grenades, although if you know what Splinter Cell is about (I'll give you a hint - guns are more useful for shooting out lights than for shooting people) you probably won't use them that much.
And if all that wasn't enough, this game also has great multiplayer. There is co-op for two players, vs mode with two new modes (disk hunt and deathmatch), and the Spies vs. Mercenaries mode returns from Pandora Tomorrow with many improvements. The co-op is seven tie-in missions to the game's main story for two players and if you can get another competitive Splinter Cell player with you, it's great fun.
In short, Splinter Cell: Chaos Theory rocks. There are enough improvements and new modes to choke a cat, and after finishing the main story there's still the co-op and multiplayer to go through. It's pretty easy to imagine someone going through the single player campaign more than once if not many times more than that as well, and the soundtrack by Amon Tobin is awesome. Anyone looking for a good stealth action game can't go wrong with this one.